1. Field of the Invention
The present invention relates to a laser optical system for scanning a laser beam using a rotary polygonal mirror made of plastics.
2. Description of the Related Art
Highly accurate smoothness and dimensional accuracy of a surface are required for a rotary polygonal mirror used for the laser beam optical scanning system. In general, rotary polygonal mirrors are manufactured by cutting aluminum materials. However, in order to reduce a load imposed on the motor by the weight and also to reduce the material cost, every effort is made in the design of rotary polygonal mirrors, that is, the rotary polygonal mirror is designed so that an effective reflecting surface of the rotary polygonal mirror on which a laser beam is irradiated corresponding to an effective scanning region on a surface of the image forming apparatus can be the same as the width in the rotary direction of the mirror surface.
However, it is difficult to machine aluminum materials accurately. Therefore, cost reduction can not be accomplished at present.
For this reason, it is recently attempted to manufacture a rotary polygonal mirror by molding plastics, however, a sufficient mirror surface accuracy can not be accomplished by the rotary polygonal mirror manufactured by means of molding plastics. Specifically, air in a metallic mold and gas generated from molten resin tend to stay at corners in the metallic mold, and there is a tendency that the surface accuracy at an end of the mirror surface is lowered.
Accordingly, when the rotary polygonal mirror made of aluminum is simply replaced with the rotary polygonal mirror made of plastic material in the laser optical system, unevenness is disadvantageously increased in the writing operation.